1. Field of the Invention
This invention relates to cardiac pacing devices and, more particularly, to programmable cardiac pacers adapted to receive external signals for re-programming of one or more pacer parameters.
2. Description of the Prior Art
A programmable pacer is one which may be altered, or programmed, so that one or more of the operating pacer parameters is changed in accordance with the new program. The clear advantage of a programmable pacer is that, at any time after implantation, such operating parameters may be changed at the direction of the patient's physician to provide more optimum operation in response to the observed condition of the patient. For example, observations may indicate that a given patient would be better adapted to receive either a higher or lower stimulus rate than that at which the pacer was set at the time of implantation. In other situations, monitoring may indicate that the threshold to stimulus of the patient has changed over a period of time to such an extent as may warrant increasing or decreasing of the stimulus strength, either to safeguard against loss of stimulus or to conserve battery energy.
Historically, programmable type pacers have been slow in gaining acceptance, for the sound reasons that doctors have been concerned primarily with reliability of operation in a pacer, and only secondarily in flexibility of adapting its operating characteristics to changes in patient condition. While the advantages of the programmable pacer have been evident, power limitations and reliability considerations have served to limit widespread use of the programmable type pacer. However, with new improved power sources, such as the lithium battery, and advanced techniques for providing long life reliability and low current drain in more complex circuits, programmable pacers are quickly coming into favor. High reliability integrated circuits and other forms of microcircuitry, in combination with the new long life batteries, permit relatively low current reliable operation of complex logic arrangements, and without any appreciable cost in terms of space, thereby rendering the programmable pacer very attractive.
Given the advantages of the programmable pacer, and presuming long life reliable operation, e.g., 8 to 12 and perhaps up to 15 or more years, the question then becomes what is the preferred manner of communicating with the long-life implanted pacer? In approaching this problem, it is understood that there is no inherent limitation on the nature of the transmitting means that is available to the physician with which to re-program the pacer, i.e., any degree of sophistication is available with respect to the external equipment used to generate signals which are transmitted through to the pacer for the re-programming activity. However, there are a number of design considerations which bear upon the choice of the transmitter and in particular the method employed for receiving re-programming signals.
One of the primary considerations in choosing a programming system is the fact that it is anticipated that the pacer may be implanted for a long period of time, for example up to 15 years. It is, of course, extremely advantageous that a pacer may be designed to be implanted for this period of time, but this recently developed advantage carries with it possible consequences which are not foreseeable. To date there is simply no experience with patients that have had a given pacer implanted for that period of time. However, it is mandatory to take notice of what is reasonably foreseeable over a 15 year period, and which could have consequences concerning a patient having an implanted pacer. For one thing, it is foreseeable that such a patient could be or likely will be mobile, i.e., he will not remain in the same geographical location. Attendant upon this fact is the consequence that his physician to whom he must go for periodic checkups may change, either because of the patient's own change of location or that of the physician, or because of the death or disability of the initial physician. For this reason it must be noted that there is a finite possibility that during the lifetime of the patient he cannot rely on the cooperation of the same physician, which physician would be presumed to have at all times the same equipment for communicating with the implanted pacer. The question then must be posed whether the patient's physician, at any given occurrence, would be certain to have the precise type of communicating equipment necessary for effective and reliable communication with the patient's particular pacer. This would, of course, be the case if there was one accepted and universal mode of communication. However, this is far from a certain prospect, in view of the large number of pacer manufacturers who are now in the commercial field, each of which is selling its own particular apparatus. For example, there are presently in the United States at least 16 pacer companies, which companies are now making available a large number of different pacer models, and there is no standardization with respect to these models such as would or could solve this particular problem. On the contrary, there is now emerging a proliferation of different pacer programming systems. The problem is even more substantial when it is appreciated that new model pacers are more frequently than not designed to have addressable uniqueness, i.e., they can only be addressed by transmission thereto of a highly complex predetermined code.
In view of these developments, from a statistical viewpoint it becomes quite possible that during the anticipated lifetime of an implanted pacer, the patient who is relying on that pacer for maintenance of his life will be examined by a physician who will not have the apparatus necessary to program all then existing models of pacers, meaning that he might not be able to program that particular patient. Stated in another way, it is statistically unlikely that 15 years from now any given physician in any given area, to whom a pacemaker patient might be expected to turn for treatment including a re-programming of an implanted pacer, would have the apparatus and the knowledge necessary to provide proper treatment to such patient. This consideration necessarily leads to a conclusion that somehow the programming system must have a simplicity that will provide maximum availability of the means for re-programming the pacer system at any time and under any circumstance.
Another concern with respect to the design of a programmable pacer is that of maintaining maximum security of the pacer with respect to leakage of fluids, dendritic growth, or any other of the multiple failure modes which have been known to occur due to the implantation of a pacer within a human patient. For pacers with anticipated lifetimes up to about 15 years, the potential for this sort of problem is increased, and any design of an additional feature such as programmability must not be accomplished at the cost of reducing the long term security of the pacer. The lithium iodide battery permits hermetic sealing of a pacer, and from a security point of view the optimum arrangement is to have a totally hermetically sealed pacer. However, such a pacer, utilizing standard materials for its casing such as titanium or other metals, would not admit of transmission of electromagnetic coding signals within the sealed pacer. Rather, for use of such electromagnetic transmission, it would be necessary to position a receiving coil or other means external to the sealed case, and then provide for a feedthrough from the receiver to the electronic circuitry within the sealed pacer. Advances in the techniques of proper sealing of feedthrough elements have been accomplished in recent years, as evidenced by the U.S. Pat. No. 4,010,759, to Boer, assigned to the same assignee. However, reduction of the number of feedthroughs to the absolute minimum remains a design objective, and in this respect a programming system which allows the pacer to have only one feedthrough, i.e., for transmission of the generated pulse signals and reception of sensed heart signals, is desirable. Additionally, the requirement of mounting a receiving element outside of the encased pacer carries the requirement of additional spaceconsuming structure outside of the pacer casing, which must be enclosed with epoxy or the like, which adds to the total size of the pacer as well as to the expense of production.
In view of the above, an optimum programming system for an implanted pacer is one which utilizes as close to a universal type transmitter as is possible, i.e., a transmitter which is so simple that statistically it is likely to be available at virtually any place and at virtually any time in the future. The one type of device which meets all of the above criteria is a simple magnet. A constant magnetic field is capable of penetrating a sealed pacer housing, such as one desirably made of titanium. It is maintenance free and has an indefinite lifetime. It is as close to being universally available in the sense that simple magnets are found in all parts of the world and can be expected to be found indefinitely into the future.
The simple use of a magnet for communication with an implanted pacer is found in the prior art. At least one prior art pacer system has been made available which utilizes the placement of a permanent magnet in the vicinity of the pacer for activating a reed switch, the simple detection of the externally applied magnetic field by the reed switch being utilized to initiate a change of a given pacer parameter in accordance with a predetermined program. However, this arrangement has at least several disadvantages. For one, security against false or extraneous programming is minimal in that any pacer patient could come into the presence of a magnetic field of sufficient strength to activate such a pacer, with unpredictable and potentially serious results. The attempt to design around this problem by requiring the magnet to be of a specific shape, or to provide a specific field strength, leads back toward the design of a more unique, and thus less universally available type of transmitter. Rather, it is desired that the pacer be adapted to respond securely to any type of simple magnet.